Purification of crude terephthalic acid by hydrogenation over a suitable catalyst is well-known. Hydrogenation offers the easiest route for removal of 4-carboxybenzaldehyde (4-CBA) impurity from the crude terephthalic acid (TA). This invention is directed to an improved process for the hydrogenation of crude terephthalic acid in the presence of a catalyst prepared by utilizing palladium and rhodium metals deposited upon an active carbon support from water-soluble precursors to produce a catalyst of improved activity and/or selectivity in hydrogenating 4-carboxybenzaldehyde to very low levels. Under severe test conditions, 4-carboxybenzaldehyde content is decreased to less than 100 parts per million (ppm) and p-toluic acid content increase is minimized.
Catalysts comprising a Group VIII metal of the Periodic Table upon an inert carrier are known for use in various hydrogenation reactions. They are usually prepared by impregnating a support material with a solution of a compound of a Group VIII metal and reducing the impregnated compound to the metal. Catalyst improvements typically have been directed to obtaining increased hydrogenation activity rather than increased activity and/or selectivity in hydrogenating specific compounds.
It is an object of the instant invention to provide an improved method for preparing a catalyst compound of a Group VIII metal. A particular object is to provide a method for preparing such catalysts having increased catalytic activity and/or selectivity in the reduction of 4-carboxybenzaldehyde. Another object is to provide a catalyst composition which comprises involving the preparation of complex salts of Group VIII metals wherein a catalyst of improved selectivity is obtained for use in reduction of 4-carboxybenzaldehyde in purification of crude terephthalic acid containing up to 10,000 ppm of 4-carboxybenzaldehyde. Still further objects will be apparent from the following specification.
The field of this invention accordingly relates to Group VIII metal catalysts for hydrogenation and purification of terephthalic acid suitable for polyester polymers and copolymers useful in the manufacture of textile fibers. These polymers and copolymers have been made by condensing terephthalic acid with ethylene glycol and other dihydric alcohols.
Such Group VIII catalysts are limited in their ability to selectively hydrogenate impurities in the terephthalic acid, especially 4-carboxybenzaldehyde. Users of terephthalic acid, such as textile fiber manufacturers, often put a rigorous limitation on the allowable concentration of 4-carboxybenzaldehyde in terephthalic acid.
As with other supported catalysts, the activity and selectivity of a Group VIII metal catalyst upon a carrier depends on numerous factors such as the amount of Group VIII metal or metals present in the catalyst, the type of support, the method by which the Group VIII metal is deposited and the distribution of the metal on the support. Improvement of Group VIII metal catalysts has typically been of activity rather than of selectivity.
Group VIII metal catalysts, such as palladium catalysts, useful in hydrogenation processes often are prepared by causing a Group VIII metal salt, such as a palladium salt, to be absorbed from a solution onto a carrier. In one procedure as is taught in U.S. Pat. No. 2,857,337, a palladium salt is thereupon treated with a water-soluble metal hydroxide or basic carbonate which is thereafter reduced to metallic palladium by reducing agents such as formaldehyde, glucose, hydrazine, glycerine and the like. Catalysts prepared by the '337 process are particularly useful in improved catalytic activity in reduction of aromatic nitro compounds to the corresponding amines.
Other conventional methods of preparing palladium catalysts have been taught. U.S. Pat. No. 2,802,794 teaches impregnation of an activated alumina support material with a solution of a compound of the platinum metal group and reducing the impregnated compound to the metal. The preconditioned activated alumina is obtained by heating a hydrated alumina to a temperature of up to 800.degree. C. whereby a microporous alumina is obtained. The resulting catalyst showed increased catalytic activity in hydrogenating butylanthraquinone to butylanthrahydroquinone.
U.S. Pat. No. 3,138,560 to Keith, et al., teaches that when sodium tetrachloropalladate or palladium chloride is added to many carbon supports, most of the palladium is immediately deposited as a shiny film of metallic palladium. Catalysts so prepared generally have low activities and it has been theorized that the palladium compound is directly reduced to palladium metal by the presence of functional groups, such as aldehydes or free electrons on the carbon surface. Palladium catalysts are accordingly taught as advantageously prepared by fixing the palladium as an insoluble compound prior to reduction to avoid the problems of migration and crystallite growth which can occur when a metal is reduced from solution. Keith '560 teaches inclusion of an oxidizing agent, such as hydrogen peroxide, to hydrolyze the palladium prior to reduction by the carbon, thus obtaining improved palladium dispersion and a highly active catalyst for the reaction of oxygen with hydrogen to give water. U.S. Pat. No. 3,288,725 to Aftandilian teaches that catalysts produced by deposition of a transition metal compound upon an inert particulate solid and subsequent reduction often have a disadvantage in that uniform deposition of the transition metal compound upon the surface of the inert particulate is accomplished with great difficulty. Hence, when the metal compound is reduced, the metal atoms deposited on the surface thereof are not exposed, are therefore not completely reduced and maximum potential catalytic activity is not achieved. Aftandilian '725 teaches that reaction of the metal compound with a particulate surface having a suitable hydroxyl group content, followed by reduction with a borohydride produces an improved catalyst for hydrogenation of unsaturates to saturates and nitro compounds to amines. U.S. Pat. No. 3,737,395 to Arnold, et al., teaches a process for preparing a catalyst which avoids formation of gels which cause lower activity. The catalysts, useful in hydrogenating unsaturated compounds such as maleic acid, are taught as having uniform and controlled deposition of palladium or platinum and a metallic promoter onto particulate carbon. An aqueous slurry is formed of the palladium or platinum compound and the water soluble metallic promoter. A precipitant is then added to precipitate the palladium or platinum and the metallic promoter, followed by co-reduction of both with a mild reducing agent such as formaldehyde, hydrazine, sodium formate, glucose or hydrogen. U.S. Pat. No. 3,271,327 to McEvoy, et al., teaches a process for depositing palladium upon the surface of a nonporous support material wherein the palladium forms a thin, firm and adherent coating, thus obtaining maximum catalytic activity by means of a thin, peripheral distribution of palladium oxide in the support material. The resulting catalyst was taught as useful in removing traces of oxygen from hydrogen by converting the oxygen to water. U.S. Pat. No. 3,328,465 to Spiegler teaches the preparation of palladium metal deposited on nonporous carbon support admixed with a porous carbon. The resulting catalyst is taught as resulting in a rate of hydrogenation of nitro compounds to amines about twice that of a hydrogenation process using the same amount of palladium deposited on a nonporous carbon. Previously, carbon used for support of palladium had been mainly porous carbon of vegetable or animal origin. Due to the high porosity of the carbon, some of the palladium became trapped in the pores and did not contribute to the activity of the catalyst. Another disadvantage was that such porous catalysts became fouled with the products of hydrogenation.
Purification of phthalic acids by hydrogenation often has been over a catalyst prepared by the above methods of palladium or platinum metal upon a support which can be porous or nonporous. However, purification of terephthalic acid by hydrogenation using typical catalysts of palladium or platinum metal prepared in the usual methods can result in unacceptable levels of impurities in the purified acid.
The impurities in crude terephthalic acid prepared by oxidation of p-xylene are partially oxidized products such as 4-carboxybenzaldehyde and toluic acid. These compounds usually are present in significant amounts in the crude acid. 4-Carboxybenzaldehyde is a particularly undesirable impurity because it acts as a chainstopper during polyesterification of terephthalic acid. Although 4-carboxybenzaldehyde is difficult to remove by physical means, it can be hydrogenated to toluic acid and other derivatives. Toluic acid also acts as a chain stopper during polymerization of terephthalic acid. However, it can be efficiently readily removed by cooling and crystallizing terephthalic acid solutions containing it. The purification of terephthalic acid therefore often has been by hydrogenation using a Group VIII metal catalyst followed by a separation process to eliminate toluic acid. An accompanying problem accordingly has been to control the activity and selectivity of the Group VIII metal catalyst to obtain lower levels of toluic acid. Accordingly, a catalyst for a process is highly desirable whereby impurities such as 4-carboxybenzaldehyde are readily and selectively substantially removed from crude terephthalic acid without increasing the level of toluic acid.
A number of techniques and processes have been developed to purify terephthalic acid by hydrogenation using palladium or platinum catalysts conventionally prepared as described above.
U.S. Pat. No. 3,522,298 to Bryant, et al., teaches a process wherein crude terephthalic acid is admixed with an inert gaseous carrier such as steam. The vapor mixture is contacted at a temperature of from 600.degree. to 1000.degree. F. with hydrogen in the presence of a catalyst such as a Group VIII metal upon a carbonaceous support, i.e., palladium upon powdered carbon. Vaporized terephthalic acid is separated by condensation from other constituents in the vapor, e.g., steam, other impurities, U.S. Pat. No. 3,542,863 to Zimmerschied teaches that hot formic acid treatment of a palladium metal on charcoal catalyst controls the activity and/or reactivity in processes where oxygenated hydrocarbons such as aromatic acids are treated under reducing conditions and initial activity of a fresh catalyst is excessive, causing over-hydrogenation of aromatic rings or carboxylic acid groups. U.S. Pat. No. 3,584,039 to Meyer teaches purification of terephthalic acid by hydrogenation in aqueous liquid phase upon a Group VIII metal on carbon in the presence of hydrogen followed by crystallization from the mother liquor. U.S. Pat. No. 3,591,629 to Stancell, et al., teaches that a phenylbenzene treated catalyst of a Group VIII metal on activated carbon particles minimizes the conversion of terephthalic acid in the presence of hydrogen while effecting high conversions of 4-carboxybenzaldehyde contaminating the crude acid to compounds readily separable from the terephthalic acid. U.S. Pat. No. 3,607,921 to Stancell teaches that contact of crude terephthalic acid with palladium on carbon support in the presence of carbon monoxide, and, alternatively, hydrogen, effects a high percentage conversion of 4-carboxybenzaldehyde contaminating the acid into substances readily separable. Surface area of the metal upon the carbon support is taught as being extremely high, to 120 square meters per gram. U.S. Pat. No. 3,726,915 to Pohlmann teaches that copper based on palladium/carbon catalysts increases the effectiveness of palladium on carbon catalysts in hydrogenation of crude terephthalic acid in selective reduction of 4-carboxybenzaldehyde. U.S. Pat. No. 3,799,976 to Nienburg, et al., teaches purification of terephthalic acid containing 4-carboxybenzaldehyde by heating an aqueous mixture of the crude acid with formic acid in contact with a Group VIII metal on carbon as catalyst. U.S. Pat. No. 4,260,817 to Thompson, et al., teaches a method for purifying crude terephthalic acid by hydrogenating the crude acid in aqueous solution to make toluic acid from 4-carboxybenzaldehyde and p-xylene from terephthalyl dialdehyde wherein the reduction takes place in two stages, the aldehyde radical forming an alcohol radical and in turn forming a methyl radical. The catalyst comprises two Group VIII metals on carbon particles.
Accordingly, it is well-known that crude terephthalic acid containing high levels of 4-carboxybenzaldehyde and other impurities can be purified by hydrogenation of 4-CBA to toluic acid over a Group VIII metal or metals on carbon catalysts. However, toluic acid levels are usually increased. Therefore, hydrogenation catalysts of improved selectivity are highly desirable. In severe evaluation tests of the invented catalysts, 4-carboxybenzaldehyde content of terephthalic acid can be reduced to less than 70 ppm and p-toluic acid content remains at a level of less than 2000 ppm.